迭代器和生成器

最简单实现的一个迭代器.

#!/usr/bin/env python
#coding:utf-8
items = [1, 2, 3]
it = iter(items)
print next(it)
print next(it)
print next(it)
print next(it)

output

python iter_and_generate.py
1
2
3
Traceback (most recent call last):
  File "iter_and_generate.py", line 10, in <module>
    print next(it)
StopIteration

自定义迭代器

只需要一个具有next方法的类,只要能够提供返回迭代器实例的__iter__特殊方法

class MyIterator(object):
    def __init__(self, step):
        self.step = step
    def next(self):
        if self.step == 0:
            raise StopIteration
        self.step -= 1
        return self.step
    def __iter__(self):
        return self
for el in MyIterator(4):
    print el

itertools模块中好玩的迭代器

islice:窗口迭代器

#!/usr/bin/env python
#coding:utf-8
import itertools
def starting_at_five():
    value = raw_input().strip()
    while value != '':
        for el in itertools.islice(value.split(), 4, None):
            yield el
        value = raw_input().strip()

tee:往返式的迭代器

1
2
3

def with_head(iterable, headsize=1):
    a, b = itertools.tee(iterable)
    return list(itertools.islice(a, headsize)), b

groupby:uniq迭代器

使用行程长度编码(RLF)来压缩数据

from itertools import groupby
def compress(data):
    reutrn ((len(list(group)), name) \
        for name, group in groupby(data)
def decompress(data):
    return (car * size for size, car in data)

生成器

使得函数需要返回一系列元素的函数变得更加简单,高效.

#!/usr/bin/env python
#coding:utf-8
def fibonacci():
    a, b = 0, 1
    while True:
        yield b
        a, b = b, a+b
fib = fibonacci()
# print fib.next()
print [fib.next() for i in range(10)]

在处理大的文件的时候，这种方式比较节省内存

关于yield,以及yeild更多姿势

#!/usr/bin/env python
#coding:utf-8
def psychologist():
    print "请告诉我你的问题？"
    while True:
        answer = (yield)
        if answer is not None:
            if answer.endswith('?'):
                print("不要想太多问题")
            elif "good" in answer:
                print("A that\'s good, go on")
            elif 'bad' in answer:
                print("Don'\t be so negative")
free = psychologist()
free.next()
free.next()
free.next()
free.send("why I show?")
free.send("good")
free.send("bad")

send()的工作机制和next（）一样。但是yield将变成能够返回传入的值。

在python3.x中，yield有更多的用法。